Tsubasa's Links
The two of them are childhood friends. They had a rift but eventually patched up.
He initially bears resentment at Tsubasa due to their class difference when they were younger. Eventually they patch up and became friends once again.
While Tsu is very tsun towards Mitty, he admires him a lot. There's no surprise they continue working close together after the years.
He serves as Tsu's mentor and older brother during their years in both 7SA and BA. There's no surprise they continue working close together after the years. Their bond is so strong that even the women they're with would get jealous of their bond